Respect write-locks in synchronous Websocket events

[philippjfr]

As described in #1148 we override the bokeh Websocket handlers asynchronous locking mechanisms to allow synchronous functions to trigger updates on the frontend immediately instead of blocking until they finish, e.g.:

import panel as pn
import time

button = pn.widgets.Button(name='Run')
progress = pn.widgets.Progress(value=0)

def update_progress(event):
    for i in range(100):
        time.sleep(0.01)
        progress.value = i

button.on_click(update_progress)

pn.Row(button, progress).show()

If we didn't do this, this example wouldn't update the progress bar until the callback finishes. However in implementing this we are not respecting the Websocket handlers write lock. This PR ensures checks whether a write lock is enabled and if so locks the synchronous events out. This ensures most computations where updates aren't triggered in very quick succession can synchronously trigger events but may result in unexpected behavior where a callback suddenly locks in the middle. However once the callback finishes it'll all be made consistent so I think this is a worthwhile tradeoff until we handle async callbacks correctly.

Fixes #1148

[codecov]

Codecov Report

Merging #1170 into master will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #1170      +/-   ##
==========================================
- Coverage   87.81%   87.79%   -0.02%     
==========================================
  Files         105      105              
  Lines       12592    12598       +6     
==========================================
+ Hits        11058    11061       +3     
- Misses       1534     1537       +3     

Impacted Files	Coverage Δ
panel/io/server.py	40.00% <0.00%> (-0.91%)	⬇️
panel/io/state.py	90.19% <100.00%> (+0.40%)	⬆️
panel/viewable.py	82.21% <100.00%> (+0.04%)	⬆️

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[mattpap]

@philippjfr, what's your policy on branch removal? I see that most (all?) are being kept after mege.

[philippjfr]

@philippjfr, what's your policy on branch removal? I see that most (all?) are being kept after mege.

There's an option to do this automatically right? Right now I manually delete them once in a while.

[mattpap]

https://help.github.com/en/github/administering-a-repository/managing-the-automatic-deletion-of-branches

[philippjfr]

Thanks, enabled.